SOCIAL PARASITISM IN BIRDS 



557 



birds become more or less promiscuous, or 

 possibly polyandrous, in their mating 

 habits . This species (and also the Scream- 

 ing Cowbird) has a very different type of 

 courtship display and song, differing in 

 this respect from the most primitive 

 species of the group. The Shiny Cowbird 

 presents a new feature, that of a differ- 

 ential sex-ratio, the males being much in 

 excess of the females, while in the first two 

 species the sexes are about equal in 

 numbers. This excess of males almost 

 seems a result of the parasitic habit, as it 

 allows for a numerical increase of the 

 species without too great an increase in 

 egg producing individuals. If too many 

 eggs were produced too few of the natural 

 foster-parents would be able to bring up 

 any of their own young and thereby pro- 

 vide an adequate supply of victims for the 

 succeeding seasons. The numerical status 

 of the parasite depends very largely on 

 those of the common host species. It 

 has been shown that in the cowbirds 

 (18) there seems to be a definite correlation 

 between the sex-ratio and testicular size 

 asymmetry. To quote from that paper: 



The condition in— (the cowbirds) — is interesting 

 and suggestive. Two of them, Agelaioides badius and 

 Molothrus rufo-axillaris, have no sexual dimorphism 

 in plumage, arc monogamous, and ratio of the sexes is 

 even, one male to one female. These two species have 

 the testes equal in size. The other three, Molothrus 

 bonariensis, Molothrus ater, and Tangavius aeneus, have 

 sexual dimorphism in plumage, are more or less 

 promiscuous with a tendency towards polyandry, and 

 the males in all three outnumber the females by at 

 least three to two, or by not less than fifty per cent. 

 In these three species the left testis is much larger 

 than the right. It is interesting to note that of the 

 five the first two (with equal testes in breeding adult 

 males) are the most primitive species of cowbirds in 

 all respects. Agelaioidcs badius is more or less normal 

 in its reproductive habits, but all the others are 

 parasitic .... The development of this habit 

 allowed for an increase of males in proportion to 

 females as true mating and pairing were probably of 

 less importance to parasitic species than to one tied 

 down by nesting and parental obligations. The 



parasitic habit is simplest in Molothrus rufo-axillaris 

 and this species still pairs off in its ancestral mono- 

 gamous fashion .... 



The Shiny Cowbird is an early breeding 

 species, and is parasitic on a great variety 

 of small birds; in fact almost all the small 

 birds breeding in the range of this cow- 

 bird are probably parasitized. Over 

 eighty species have been found caring for 

 its eggs or young. 



The North American Cowbird is very 

 similar in its habits to the Shiny Cowbird. 

 In song, courtship display, relative number 

 of the sexes, sexual relations and lack of 

 specificity of host species, the two are very 

 similar. However, the Shiny Cowbird has 

 the parasitic habit less well developed and 

 wastes large quantities of its eggs, either 

 by laying them on the ground and leaving 

 them there or by laying too many eggs in 

 one nest or in deserted nests, etc. The 

 North American bird is more efficient in 

 the disposition of its eggs, and wastes 

 relatively very few of them. About two 

 hundred species of small birds are known 

 to be parasitized by this cowbird. 



The Red-eyed Cowbird represents an- 

 other branch of the cowbird tree and is 

 more similar to the Screaming Cowbird 

 than to any other. It is parasitic on sev- 

 eral species, usually birds of genera fairly 

 closely related to itself, such as Icterus. 

 The males somewhat outnumber the 

 females, and in general the relations of the 

 sexes are similar to the condition in the 

 North American Cowbird. 



THE BREEDING AREA 



One other topic demands our attention 

 before we can piece together the various 

 bits of evidence offered by the different 

 species of cowbirds. This is the matter 

 of the individual breeding area or territory. 

 Howard (jx) has shown that normally 

 each pair of birds establishes an individual 



